Development of Novel Mixed Matrix Asymmetric Membranes for C02 Separation from Natural Gas

The use of membrane technology for gas separation applications has been
successfully employed since the last few decades. For efficient separation of C02
from natural gas, high performance gas separating membranes is desired. For
C02/CIL. gas separation, asymmetric flat sheet membranes are preferred because of
their high gas permeance as compared to dense films. In this study, various
composition of polyimide (PI) ranging from 5 to 20 wt.% were blended with
polysulfone (PSF) and used for the formation of asymmetric flat sheet membranes via
dry/wet phase inversion technique.
The performance of asymmetric membrane was further enhanced by incorporating
inorganic silica of tetraethyl orthosilicate (TEOS) at various proportions ranging from
5-20 wt.% into the membrane system containing 20 wt.% PI (PSF/PI-20wt%) to form
mixed matrix membranes (MMMs). All the membranes prepared were characterised
using scanning electron microscopy (SEM), thermogravimetric (TGA), differential
scanning calorimetry (DSC), fourier transform infrared spectroscopy (FTIR),
transmission electron microscopy (TEM), mechanical analysis and followed by gas
permeation analysis.
Morphological analysis indicated that the surfaces of the fabricated membrane
blends possessed homogenous surfaces and their cross-sections showed a non-porous
top and a diminutive porous substructure. DSC analysis showed the existence of
single glass transition temperature (T g) for different membrane blends which indicated
miscibility among the polymeric blends. Mechanical analysis showed improvement in
young's modulus, tensile strength and elongation at break properties with the increase
in PI composition in the membrane blends. Solvents with various compositions ofN�methyl-2-pyrrolidone to dichloromethane (NMP/DCM) were used to prepare the
membranes and it is found that 80/20 v/v% solvent composition offered maximum
C02/CH4 gas separation performance. Heat treatment was carried out on the membranes prepared from the NMP/DCM (80/20) solvent composition which showed
improvement in ideal selectivities with a slight decrease in the permeance for all the
membranes. Kinetic analysis on the thermal degradation of the developed membranes
was also studied using TGA analysis by Friedman's model approach. The thermal
analysis showed improvement in the degradation temperature and increase in
activation energy values with an increase in the PI contents in the PSF/PI membrane
blends.
The developed MMMs showed different morphologies of the surfaces and cross�sections of the membrane where agglomeration was observed at 20 wt.% silica
loading. The DSC analysis showed the existence of a single T g value and it increased
with the increase in the silica loadings in the MMMs. The XRD analysis showed a
decrease in the d-spacing with the increase in silica loadings causing restriction in the
polymer chain mobility. The kinetic analysis on the thermal degradation showed an
improvement in the thermal stability and the activation energy increased with the
increase in the silica contents. Mechanical analysis indicated a steady increase in
Young's modulus and tensile strength up to 15 wt.% silica loading. Elongation at
break decreased with an increase in the silica contents which indicated the rigidity of
the MMMs. The gas permeation results showed that the C02 permeance increased
from 73.7±0.2 GPU at 5 wt.% silica content to 95.7±0.4 GPU at 20wt.% silica
content. However the maximum ideal selectivity, aco,ICH, of 61.0-60.2 at 2-10 bar
feed pressure is observed at 15wt.% silica content. The selectivity using mixed gas
analysis at various C02/CH4 compositions of 30/70 v/v%, 50/50v/v% and 30/?0v/v%
showed consistent results with the ideal gas selectivity.
Finally, various theoretical gas permeation models for predicting C02 gas
permeance in the MMMs were applied. A closer look revealed that the existing
models assumed the spherical shape of the fillers dispersed in the matrix; however,
later investigations by SEM indicated the fillers shape to be actually prolate
ellipsoids. It was observed that the percentage of average absolute relative error
(AARE %) values obtained from the MWS model were found to be in the range of
1.12-2.17 at 2-10 bar. Further investigations from the shape factor in the z-direction
(nL) and the estimated shape factor (n,), AAR% deviations obtained were in the order of nr <n, <nz ; this indicated the importance of shape factor parameter for estimating
true C02 permeance.